In integrated circuits of 130 nm or less technology, the transistors exhibit a not-inconsiderable leakage current when they are in the open state. The result of this is electrical consumption in the circuits even when they are inactive. This consumption can be very problematic, notably for circuits powered by cells or batteries, above all when, for most of the time, they are in a standby more or only a small part of the circuit is active. This is particularly true for portable telephones.
One effective technique for reducing this leakage current when the circuit is inactive consists in cutting the electrical power supply to this circuit. Other techniques, notably the one consisting in supplying the substrate or chamber of CMOS circuit with a negative voltage in order to increase the threshold voltage of the NMOS transistors, are possible but more difficult to implement.
One problem in the technique involving cutting the power supply is that, when the power is restored, the inrush current created by the recharging of the capacitance between the power supply gates of a switched block must not cause the power supply voltage to drop upstream of the switch in order to avoid affecting the operation of the other blocks. It is therefore best to limit this inrush current on powering up.
Various solutions are known for limiting this current. In particular, one solution consists in producing a power supply switch consisting of several transistors in parallel with progressive switching of these transistors using a series of delaying circuits. Such a solution is notably described in the document “Techniques for Wireless Applications by Philippe Royannez et al., ISSCC2005, paper 7.6, pages 138 and 139”.
In both cases, this switching time is determined by simulation and a significant margin must be taken to take account of the dispersion of the characteristics of the transistors. Because of this margin, the time taken to power up again may be lengthy, preventing these solutions from being used for power-down durations that are too short or in the case where a rapid response from the system is necessary.